Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Consequently, the use of exhaust aftertreatment systems on engines to reduce emissions is increasing.
Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (e.g., diesel) engines typically monitor the release of carbon monoxide (CO), unburned hydrocarbons (UHC), diesel particulate matter (PM) such as ash and soot, and nitrogen oxides (NOx).
With regard to reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) systems, are utilized to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, to reduce the NOx. Currently available SCR systems can produce high NOx conversion rates allowing the combustion technologies to focus on power and efficiency. However, currently available SCR systems also suffer from a few drawbacks.
SCR systems utilize a reductant delivery system to introduce a reductant, such as aqueous urea or diesel exhaust fluid, into the exhaust stream upstream of the SCR catalyst. The reductant may tend to form deposits over time within the exhaust aftertreatment system, such as on exhaust aftertreatment components (e.g., reductant dosers) and/or on the walls of exhaust conduits (e.g., reductant decomposition chambers). Such deposits can adversely affect the operation of the engine (e.g., by restricting the exhaust flow passageway) and the exhaust aftertreatment system (e.g., by impeding the catalytic reaction). If the deposit is not soon detected and remedied, the engine system may not function properly. For example, reductant deposits may negatively affect fuel consumption, NOx reduction efficiency, and other operating characteristics of an internal combustion engine.